Lately, a hybrid vehicle, which has an internal combustion engine and a motor generator(s) as drive sources of the vehicle, attracts attention because of increased public demands of low fuel consumption and low exhaust emissions. For example, JP H07-135701A teaches a hybrid vehicle, which has an internal combustion engine and first and second motor generators. A drive force of the engine is divided to two systems through a planetary gear mechanism. An output of one of the systems is used to drive a drive shaft to drive wheels of the vehicle. Furthermore, an output of the other one of the systems is used to drive the first motor generator to generate an electric power. The electric power generated by the first motor generator and/or electric power supplied from a battery is used to drive the second motor generator to enable driving of the drive shaft with the power supplied from the second motor generator.
In the hybrid vehicle, which has the engine and the two motor generators, it is demanded to achieve three objectives, i.e., (1) controlling of the rotational speed of the engine, (2) controlling of the output torque and (3) limiting of input and output of the electric power at the battery. However, even though the torque of the first motor generator and the torque of the second motor generator are controlled to achieve the above discussed three objectives, the following incident may possibly occur. Specifically, in a case were the torque of one of the first and second motor generators is limited (e.g., by a guard value at a guard process) to prevent occurrence of overheating of the one of the first and second motor generators, a desired torque (a demand torque or a specified torque) of the one of the first and second motor generators may not be outputted. In such a case, all of the above-discussed three objectives may not be achieved.